An anti-reflection thin film is capable of enhancing the transmittance of an optical device so as to reduce unnecessary reflections and glare, which possesses an important application value and development prospect in such fields as solar cell, liquid crystal display and optical element, etc. The basic principle for anti-reflection is to achieve the purpose of anti-reflection by means of interference and counteraction of light under certain conditions (the anti-reflection mechanism is as shown in FIG. 1), wherein when the light resulting from incident light being reflected at the surface of an anti-reflection layer and the light resulting from the incident light being reflected at the interface between the anti-reflection layer and a substrate are destructed due to the reversal of their phases under certain conditions, the energy of the reflected light can thus be counteracted. For an ideal mono-layer anti-reflection thin film, it must meet the following two requirements: the optical thickness of the film, i.e., the product of the thickness of the thin film and the refractive index thereof, is one quarter of the wavelength of the incident light; the square of the refractive index (n1) of the film is required to be equal to the product of the refractive index (n2) of the substrate and the refractive index (n0) of air, namely, n0n2=n12. Generally, n0 is 1, while commonly used substrates of quartz, glass and some transparent polymers have a refractive index in the range of from about 1.45 to 1.53, and so n1 is required to be in the range of from about 1.21 to 1.24. However, the current dielectric materials have the lowest refractive index of about 1.35 and cannot meet the requirements on the ideal mono-layer anti-reflection thin film. It is well-known that porous materials have a relatively low refractive index, and in recent years there appeared a variety of research methods for the preparation of a porous anti-reflection thin film, such as etching, sol-gel, vapor deposition, microphase separation, and particle impregnation film forming, etc. However, the preparation processes of these methods are relatively complicated, time-consuming and costly, and the pores in the porous anti-reflection thin film as prepared mostly have an open-celled structure, the mechanical performance and scrub resistance of the thin film need to be further improved.
A miniemulsion is a kind of dynamically stable liquid-liquid dispersion system, the dispersed droplets have a size that can be adjusted between 30 and 500 nm, and the monomer droplets in the miniemulsion polymerization can be directly converted into emulsion particles, i.e. a monomer droplet nucleation mechanism. Accordingly, the monomer droplets in the miniemulsion system can be regarded as nano-reactors independent from each other, which are very suitable for preparing nanoparticles with various structures. The core of reversible addition-fragmentation chain transfer living radical polymerization (RAFT living polymerization for short) lies in introducing a chain transfer agent called a reversible addition-fragmentation chain transfer agent into the radical polymerization system, which usually is dithioester or trithiocarbonate, wherein the radical can be subjected to an efficient reversible chain transfer reaction with the reversible addition-fragmentation chain transfer agent such that the polymer chain has active features. The present invention introduces an amphiphilic macromolecule reversible addition-fragmentation chain transfer agent into the miniemulsion system in combination with the monomer droplet nucleation mechanism of the miniemulsion polymerization and the features of the reversible addition-fragmentation chain transfer living radical polymerization. Since the amphiphilic macromolecule reversible addition-fragmentation chain transfer agent has an amphiphilic structure, it not only can be auto-assembled at the monomer droplet interface, but also plays the role of the reversible addition-fragmentation chain transfer agent to achieve the reversible addition-fragmentation chain transfer living radical polymerization, such that the monomer and a crosslinking agent can be crosslinked and polymerized at the monomer droplet interface to form highly crosslinked polymeric shells, the core material undergoes a phase separation from the polymer and is located at the center of the particles to form polymer nanocapsules with a complete structure. When the shell of the polymeric nanocapsules consists of highly crosslinked polymers, hollow polymeric nanoparticles with a high strength can be obtained after the core material thereof has been removed, which can maintain a relatively regular spherical structure while do not suffer from deformation and collapse.
When the cavity volume of a hollow polymeric nanoparticle has a diameter of less than 100 nm, a thin film composed of the hollow polymeric nanoparticles is transparent, and the cavity volumes of the hollow nanoparticles can effectively decrease the refractive index of the thin film, thereby forming a porous thin film with a low refractive index. The present invention prepares highly crosslinked polymeric nanocapsules having a diameter in the range of from about 80 to 120 nm in the miniemulsion system by means of the reversible addition-fragmentation chain transfer living radical polymerization, forms a thin film by spin coating an aqueous dispersion of polymeric nanocapsules on the surface of a substrate, and then obtains a transparent porous anti-reflection thin film composed of hollow polymeric nanoparticles after removing the core material of the polymeric nanocapsules by drying in a vacuum at a high temperature. The present invention requires simple preparation processes and can effectively adjust the thickness and refractive index of the anti-reflection thin film by changing the concentration of the aqueous dispersion of polymeric nanocapsules and the cavity volume fractions of the hollow polymeric nanoparticles so as to meet different anti-reflection requirements. In addition, when the mass percentage concentration of the aqueous dispersion of polymeric nanocapsules is not less than 5%, most of the pore structures in the porous anti-reflection thin film formed by hollow polymeric nanoparticles being densely aligned are closed cell structures consisting of cavity portions of the hollow polymeric nanoparticles, and the backbone thereof consists of highly crosslinked polymers. Consequently, the formed porous anti-reflection thin film has a relatively high mechanical strength and friction resistant property, which can effectively overcome the problem of a poor mechanical performance existing in the current porous anti-reflection thin film.